Lithium ion secondary batteries that use a carbon material as a negative electrode active material, that use a lithium-containing composite oxide as positive electrode active material, and that use as an electrolytic solution an aprotic solvent to which an electrolyte has been added have been receiving attention as power sources for use in portable telephones or notebook personal computers due to their ability to realize high-energy density. An electrolyte that is added to an electrolytic solution is also referred to as a supporting electrolyte. The negative electrode of a lithium ion secondary battery is formed by taking a simple substance carbon powder such as graphite or amorphous carbon as the negative electrode active material, forming a slurry in which this simple substance carbon powder is dispersed in a binder and an organic solvent then added, applying this slurry to a collector formed from, for example, metal plates, and then removing the organic solvent by vaporization. A material such as polyvinylidene difluoride (PVDF) is conventionally used as the binder.
However, when a lithium ion secondary battery is initially charged after assembly, gas forms within the electrolytic solution and a protective film with lithium ion conductivity referred to as a SEI (Solid Electrolyte Interphase) film forms on the negative electrode surface. Gas remaining inside the cell causes degradation of the characteristics, and the occurrence of gas can interfere with the formation of a uniform protective film. If a stable protective film is not formed, the capacity of the battery is greatly reduced with repeated charging and discharging.
In JP H11-111339A [PL1], disclosed is a matter of forming a film while suppressing the generation of gas by maintaining a pressurized state inside the battery when initially charging the lithium ion secondary battery, and after charging the battery to the charge completion voltage, releasing the pressurized state and then sealing the opening in the battery receptacle while under normal pressure or reduced pressure. JP 2000-277144A [PL2] discloses inserting battery elements in an aluminum-laminated bag, infusing an non-aqueous electrolyte solution and then sealing the bag opening, followed by charging until a predetermined battery voltage is generated to complete the initial generation of gas, then carrying out a charging process for only a necessary amount of electricity, holding this charged state as is in a high-temperature environment for a necessary time interval to cause generation of gas from the electricity-generating elements, opening a portion of the bag in the hot environment to exhaust the gas that has accumulated inside, and then resealing the bag.
In the initial charging of the lithium ion secondary battery, a widely followed procedure includes first implementing a preliminary charging step of carrying out preliminary charging to produce gas inside the cell; then implementing a degassing step of opening the outer sheathing material of the cell to exhaust gas to the exterior, resealing the outer sheathing, and then implementing a main charging step of charging up to the prescribed fully charged voltage.
In a lithium ion secondary battery, constant-current constant-voltage charging is typically used as the charging method both because excessive voltage and over-charging must be strictly avoided and because voltage of terminals increases with increasing charging capacity. In constant-current constant-voltage charging, constant-current charging is first carried out at a prescribed current value while the voltage of cell terminals is monitored, and when the terminal voltage reaches the set voltage, constant-voltage charging is carried out at the set voltage. Even when initial charging is carried out by executing the preliminary charging step, degassing step, and main charging step, constant-voltage constant-current charging is typically carried out for each of the preliminary charging step and main charging step.
Regarding the protective film on the surface of the negative electrode, JP 2002-203609A [PL3] discloses the provision of a step of forming a protective film on the surface of the negative electrode by carrying out constant-current charging and then carrying out constant-voltage charging to cause a decomposition reaction in the non-aqueous solvent of a non-aqueous electrolyte, and a step of carrying out charging for absorbing lithium into the negative electrode.
In order to form a stabilized protective film that is formed on the negative electrode surface and positive electrode surface in a lithium ion secondary battery, processes have been proposed in which an additive that can be decomposed by a predetermined voltage is added to an aprotic electrolyte solution, following which the protective film is formed by a decomposition reaction of the additive at the time of initial charging. JP 2006-351332A [PL4] discloses adding a chain disulfonate ester to an electrolytic solution and then carrying out charging in a temperature range of 30 to 60° C. JP 2011-054408A [PL5] discloses carrying out constant-current constant-voltage charging in which the set voltage is 3.8 to 4.1 V in a lithium ion secondary battery that uses an electrolytic solution that contains fluorinated cyclic carbonate as an additive, followed by degassing.